The object of this research is to determine the range of variation of certain phenomena of heart rate, rhythm and conduction, which occur in middle-aged men under the conditions of daily life, and the relation of these to coronary heart disease and to sudden death. Since 1962, samples of 20,000 to 130,000 cardiac complexes have been obtained from 940 actively employed men aged 30 to 65 by recording their electrocardiograms over six to twenty-four hour periods during carefully standardized activities. These men have also been studied in terms of various medical, physical, biochemical, behavioral and environmental factors thought to be relevant to coronary heart disease and sudden death, and have been followed prospectively over periods up to twelve years with all deaths and illnesses investigated, and with re-examinations at various intervals. Recordings are being analyzed for abnormalities of rate, rhythm and conduction and these are being studied in relation to present evidences of disease and the subsequent development of heart disease and sudden death. Recent findings indicate that abnormalities of rate, rhythm and conduction all increase risk of death in ambulatory men who have coronary heart disease, hypertensive heart disease and congestive heart failure. Risk increases with frequency of VPCs and with early cycle VPCs.